Sketching helps in figuring out possible solutions for the problem we are trying to solve. While we sketch, new ideas and different solutions emerges which can be evaluated with the previous one for the better result. Initially it starts with diverge ideas which are then converged to single sketch for a screen after lot of iterations. We can also have the UI flows once we finalize the sketch for each screen. It's kind of brainstorming experience. It is one of the efficient tool to involve stakeholders early on in the design process.